The balance between different cell populations in the developing organism is controlled by regulating the rates of multiplication, differentiation or death of its constituent cells. The human teratocarcinoma derived cell line Tera 2, which in several aspects mirrors early embryonic cells, can be induced to undergo programmed cell death (apoptosis) by depriving cell cultures of serum. This study demonstrates that this process can be reversed by replacing serum with physiological concentrations of insulin like growth factor I (IGF I). As a result, IGF I enhances the rate of Tera 2 cell proliferation in serum free medium. In contrast, Transforming Growth Factor beta1 did not exert any effect on growth or apoptosis in Tera 2 cells. The results indicate that one effect of growth factors on pluripotential cells is to regulate the balance between cell proliferation and cell death.